Quality of Earth Science data products based on observations from spaceborne radiometric sensors depends on their performance and accuracy on orbit. The accuracy of measuring reflected solar radiance can be affected by multiple factors. First, instruments with complex optics are sensitive to polarization. The response of such instruments is characterized before the launch, however, sensitivity to polarization can change on orbit significantly (e.g. Moderate Resolution Imaging Spectroradiometer (MODIS) launched on board the Terra satellite). Other factors are the degradation of optics, particularly in blue wavelength range below 500 nm (e.g. Clouds and Earth's Radiant Energy System (CERES) launched on board the Terra & Aqua satellites), and on-orbit changes in the instrument response to stray light. None of the existing sensors has the ability to monitor all these changes in calibration on orbit.
Accurate verification of space born sensors calibration on orbit plays a crucial role in meeting mission accuracy requirements. Onboard verification systems significantly impact mission costs by increasing the mass of instrumentation and required power. Also, onboard verification systems are not accessible for adjustment, maintenance, improvement, or repairs in the case of failure. Accordingly, a need exists for an improved calibration concept that does not suffer from the drawbacks of known calibration systems and methods.